This invention relates to food preservatives to prevent the discoloration of physiologically injured fruits and vegetables, and is more particularly related to a food preservative which is effective to prevent the discoloration of cut potatoes prior to serving.
When a plant material such as fruit or vegetables is cut or physiologically injured, a brown or black discoloration appears at the site of the cut or injury within a few minutes. The discoloration of fruits and vegetables as a result of cutting and other preparation for human consumption is a problem of great economic importance in the food industry. The discoloration of such vegetable matter is unsightly and unappetizing, and is associated by the public with distastefully old food. This problem has become more acute with the popularity of fresh salad bars, commonly found at fast food outlets and restaurants in all price ranges. In order to satisfy the demand for fresh fruits and vegetables at salad bars, it is frequently necessary to cut the fruits and vegetables to edible size many hours before they may be consumed. The discoloration of such pre-prepared fruits and vegetables has required that they be treated to prevent their discoloration so that the restaurant customer will perceive an appetizing selection at the salad bar. The discoloration problem has been the subject of much study because of its great economic importance in the food industry. In fruits and vegetables, tyrosine or one of its oxidation products are the substrates most often implicated in the blackening of the injury site. The browning reaction of injured fruit and vegetables is more often attributed to o-hydroxyphenols, such as chlorgenic acid. Phenol oxidase or polyphenol oxidase enzymes have been reported to cause the discoloration in peaches, grapes, pears, avocados, apples and potatoes.
In 1979, it was reported that the total annual potato market was as much as 2.4 billion pounds. Because of their importance as a foodstuff, particular attention has been directed to the problem of discoloration of potatoes when they are cut. Both chlorgenic acid and tyrosine are present in potatoes, and tyrosine is thought to be responsible for the enzymatic blackening of potatoes.
The enzymatic blackening of potatoes and the effect of inhibitors of enzymatic oxidation were reported in "Comparisons of Inhibitors of Tyrosine Oxidation in the Enzymatic Blackening of Potatoes", Muneta, P., 1981, Am. Pot. J. 58, 85. Muneta indicates that the blackening of potatoes results from the enzymatic oxidation of tyrosine by polyphenol oxidase, a coppercontaining enzyme, and that in the presence of oxygen, the enzyme oxidizes tyrosine to 3,4,-dihydroxyphenylalanine (dopa), which is then rapidly oxidized by the enzyme to dopaquinone; and further that the dopaquinone cyclizes to 5,6,-dihydroxyindole derivatives which are oxidized to a reddish-brown orange "dopachrome" pigment which is seen in the early stages of enzymatic blackening. After the dopachrome formation, a series of non-enzymatic polymerizations, oxidations and reactions with protein form brown to purple, and finally the black malanin pigment. Enzymatic browning in fruit usually involves oxidation of o-hydroxyphenols such as chlorgenic acid instead of tyrosine.
Muneta reports that the chemical control of enzymatic blackening or browning involves the inhibition of the polyphenol oxidase activity by adjusting the pH, adding bisulfite or sulfhydryl compounds; the chelation of the copper from the polyphenol oxidase; the use of reducing compounds which reduce the wholequinones to the o-hydroxyphenol state; or chemicals which react with o-quinone to give colorless addition products.
Muneta tested and reported the effects of chemical inhibitors on the enzymatic oxidation of tyrosine using polyphenol oxidase obtained from potatoes. Muneta removed the polyphenol oxidase enzyme from potatoes by acetone precipitation, dialyzed and buffered the solution and measured the oxygen uptake to determine the rate at which the polyphenol oxidase enzyme oxidized a tyrosine substrate. As a result of his testing, Muneta reported that bisulfite was a very effective inhibitor of tyrosine oxidation. Muneta also indicated that cysteine, dithiothreitol and two sulfhydryl compounds temporarily inhibited the blackening. Muneta found that partial inhibition required twenty times the concentration of cysteine as dithiothreitol on a molar basis.
Muneta reported, however, that potatoes pose special problems because of the natural occurrence of o-hydroxyphenol such as chlorgenic acid, along with the tyrosine. The chlorgenic acid, without sufficient bisulfite and sulfhydryl compounds present, is oxidized to quinone which reacts with the inhibitor to reduce the effective inhibitor concentration, and permits the tyrosine oxidation to proceed. Thus, in order to inhibit the melanin blackening formation in the presence of chlorgenic acid and tyrosine, higher inhibitor concentrations are required.
Muneta also tested the effect of ascorbic acid on tyrosine oxidation, and reported that oxygen uptake was more rapid in the presence of ascorbic acid than when only tyrosine was oxidized. Muneta noted that the ascorbic acid inhibited blackening only temporarily by reducing the dopaquinone back to dopa.
Muneta summarized that from a food processor's standpoint, bisulfite is preferred to other chemical inhibitors, that cost and flavor problems are deterrents to the use of sulfhydryl groups such as dithiothreitol and cysteine, and that inhibitors such as ascorbic acid are not desirable because blackening will occur when the ascorbic acid is oxidized.
Although bisulfites have proven to be the most popular and widely used preservatives for fresh fruits and vegetables, the United States Food and Drug Administration has recently banned their use on fruits and vegetables including the use in restaurant salad bars effective August 8, 1986.
Other studies have been made of the problem of discoloration of fruits and vegetables. Among the studies, there is one reported by Cheryll Reitmeier and R. W. Buescher in Arkansas Farm Research, Vol. 24, 1975. The study by Reitmeier and Buescher indicated that carbon dioxide atmospheres of 10 to 20 percent could reduce the black and brown discoloration of snap beans at 75 degrees Fahrenheit and effectively prevent it for 48 hours at 60 degrees Fahrenheit, and suggested shipping snap beans with dry ice in commercial systems to both reduce the temperature and increase the carbon dioxide levels.
Reitmeier and Buescher also studied the effect of chemical materials in delaying the rate and intensity of the browning discoloration. Snap beans, which were washed and cut into sections, were sprayed or washed with solutions for 1 to 30 seconds. Browning discoloration was then evaluated on the residue from cut end tissues after the tissues had been homogenized and centrifuged. Reitmeier and Buescher tested a number of materials on the prepared bean residue. Citric acid in a 5 percent concentration was reported as providing a 67 percent inhibition after 24 hours, but a 13 percent inhibition after 48 hours. Cysteine in a 0.02 percent concentration was reported as demonstrating 100 percent inhibition after 24 hours, but 50 percent inhibition after 48 hours. Treatment with enriched carbon dioxide atmospheres and low temperatures was preferred because of the clearances needed for chemical treatments.
Commercially available food preservatives include compositions combining citric acid, calcium chloride, erythorbic acid, tricalcium phosphate, monocalcium phosphate; and materials for preventing the conversion of starch to sugar in sweet corn such as those disclosed in U.S. Pat. Nos. 3,837,837 and 3,876,412.